Electrochemical cells containing an anode, a cathode and a solid, solvent-containing electrolyte are known in the art and are usually referred to as "solid batteries." These cells offer a number of advantages over electrochemical cells containing a liquid electrolyte (i.e., "liquid batteries") including improved safety features.
In solid batteries the solid electrolyte is interposed between the cathode and anode. The solid electrolyte contains either an inorganic or an organic matrix as well as a suitable salt. The inorganic matrix may be non-polymeric, (e.g, .beta.-alumina, silicon dioxide, and lithium iodide), or polymeric, (e.g., inorganic (polyphosphazene) polymers), whereas the organic matrix is typically polymeric. Suitable organic polymeric matrices are well known in the art and are typically organic polymers obtained by polymerization of a suitable organic monomer as described, for example:, in U.S. Pat. No. 4,908,283. Suitable organic constituents include, by way of example, polyethylene oxide, polypropylene oxide, polyethyleneimine, polyepichlorohydrin, polyethylene succinate, and an acryloyl-derivatized polyalkylene oxide containing an acryloyl group of the formula CH.sub.2 =CR'C(())O-- where R' is hydrogen or lower alkyl of from 1-6 carbon atoms. Because of the expense and difficulty in shaping inorganic non-polymeric matrices into the desired configurations, solid electrolytes containing polymeric matrices are preferred.
The solid electrolytes may also contain a solvent (plasticizer) which is typically added to the matrix in order to enhance the solubility of the inorganic salt in the solid electrolyte and thereby increase the conductivity of the electrolytic cell. Suitable solvents well known in the an for use in such solid electrolytes include, by way of example, propylene carbonate, ethylene carbonate, .gamma.-butyrolactone, tetrahydrofuran, glyme (1,2-dimethoxyethane), diglyme, triglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane and the like.
Solid hybrid electrolytes are heterogeneous multi-phase electrolytes containing at least one ion conducting phase. A hybrid electrolyte, suitable for electrochemical cells, comprises a liquid phase and a solid polymeric phase. In most electrolyte systems that employ organic solvents, the salt is usually not completely dissociated. This is due to a combination of low permitivity of the solvent and the relatively high salt concentration, which is required in many applications such as batteries and super capacitors. When the salt is not fully dissociated, a series of equilibria will exist in which associated species are formed. Those species can be ion-pairs, triplets, and even larger clusters of ions. They may all be mobile in the electrolyte and contribute to the charge transport. In batteries, for example, the electrodes are active only towards one of the ion constituents and blocking towards the other. Specifically, in alkali, secondary batteries the cation is the electrode active species.
The transport number of an ion in a given electrolyte solution is the fraction of the total electrical current carried in the solution by that ion. Every species in the electrolyte has a transport number t.sub.i, which must always be positive and between zero and one as: ##EQU1##
Where ion association occurs, it is impossible to distinguish between simple ions and other charged species. Instead the total material transfer or total current is determined. Therefore, an observable quantity relies on the transfer of an ion constituent and not a free ion. This quantity is called the transference number. Because the transference number is based on the transport of the gram-equivalent of one faraday, it follows that: ##EQU2##
In a conventional solid electrochemical cell, the cations and anions in the electrolyte are evenly distributed throughout the electrolyte when the cell is not generating any current. When a current is being generated and both the anions and cations are mobile (each, for instance, with a transference number of 0.5), then half of the current through the electrolyte is transported by the cation and the other half by the anion. In most systems, only the cation react/intercalate with the electrodes, so eventually there is an accumulation of anions at the electrode, which is discharged. (The rate of accumulation depends on the electrolyte composition, the current density, and the electrolyte thickness.) Since the amount of negative and positive charges in a given volume in the discharged is equal the electrolyte is polarized. This will lead to a number of phenomena. In the steady state situation all the current will have to be carried by the cation, so the conductivity of the electrolyte drops to half of its original value. Furthermore, a salt concentration gradient will develop in the electrolyte. This will further increase the resistance of the electrolyte, which leads to both a reduction in power capability and effectiveness of the battery. In the case of recharging a lithium metal anode battery, uneven plating will occur, which leads to a shorter cycle life. For a discussion of transference number, see F. M. Gray, "Solid Polymer Electrolytes" (1991), pp. 193-94, VCH Publishers, Inc.
In view of the above shortcomings associated with prior art solid state electrochemical devices, there is a need for solid electrolytes that exhibit reduced polarization and electrochemical cells that have improved cycle life, capacity.